Coextrusion die

ABSTRACT

A coextrusion die for the extrusion of plastics materials in the shape of a tubular film comprising supply means for separately supplying at least two plastics materials to said die, a distribution channel system connected to each supply means, each distribution channel system comprising at least two arched channels provided in different axial planes, the ends of each arched channel being connected to the middle portion of the following arched channel and the distribution channel systems being located at different distances from the center axis of the die, collecting chambers communicating with said distribution channel systems and an annular extrusion slot communicating with the collecting chambers.

This is a division of Ser. No. 314,070, filed as PCT DK81/00023, Feb.27, 1981, published as WO81/02406, Sept. 3, 1981, §102(e) date filedOct. 20, 1981, now U.S. Pat. No. 4,403,934, issued Sept. 13, 1983.

TECHNICAL FIELD

This invention relates to an extrusion die for the coextrusion ofplastics materials in the form of a tubular film, said apparatuscomprising means for separately supplying at least two extrudablematerials in fluid state, a system of distribution channels connected toeach supply means to form partial streams of said extrudable materials,each system of distribution channels opening into an annular collectingchamber, and an annular extrusion slot communicating with said annularcollecting chamber(s).

BACKGROUND ART

In a prior art coextrusion die of the above mentioned type eachextrudable material is supplied in liquid state through a centrallylocated supply conduit which is connected to a system of radiallyextending distribution channels, each system being provided in adistribution plate, and is then passed into an annular collectingchamber from which it is passed into a common annular extrusion slot.

Due to the presence of the radially extending distribution channels inthe prior art coextrusion there is insufficient space for the strongclamping bolts which are required to maintain the distribution platessufficiently tensioned to avoid leakage of extrudable materials betweensaid plates at high extrusion pressures.

The prior art supply system for extrudable materials comprisingcentrally located supply conduits also suffer from the drawback that itallows no space at the centre of the die for the provision of drivingmeans when it is desired to rotate the inner and outer die partsrelative to one another or for the provision of supply and dischargeconduits for gaseous coolant, such as air to cool and expand theextruded tubular film. In the absence of such cooling, the productionrate is considerably reduced.

The object of the invention is to provide a coextrusion die providingsufficient space for the provision of clamping bolts, means for rotatingone of the die parts relative to one another and supply and dischargeconduits for gaseous coolant.

DISCLOSURE OF INVENTION

According to the invention there is provided an extrusion die whereinthe distribution channel system for each extrudable material comprisesat least two arched channels provided in different axial planes, theends of each arched channel being connected to the middle portion of thefollowing arched channel and wherein the distribution channel systemsare located at different distances from the centre axis of the extrusiondie.

In the following the invention will be described with reference to anextrusion die for the coextrusion of three extrudable materials, whereinthe arched distribution channels are circular arches and wherein thedistribution channel systems are located concentrically because the useof such distribution channels and the concentric mounting of thesesystems are particularly advantageous as far as the manufacture of theextrusion die is concerned.

However, it should be understood that the die may comprise two or morethan three distribution channel systems and that the arched distributionchannels should not necessarily have the shape of circular arches andeven may be composed of linear channel sections and that thedistribution channel systems do not have to be concentrically mounted.

The invention is based on the discovery that by effecting thedistribution of each extrudable material in a labyrinthic manner indifferent distances from the centre axis of the die and in differentaxial planes thereof, it is possible to avoid conduits for supplyingextrudable materials to the die and radially extending distributionchannels in the central zone of the die. Thus, the supply conduits canbe connected to the end surface or exterior surface of the die and canbe connected to the first arched channel of each distribution channelsystem through connection channels provided in the die.

Thus, the necessary clamping bolts can be inserted between thedistribution channel systems and driving means, if any, for the rotationof the inner die part as well as conduits for gaseous coolant can bemounted in the central zone of the die.

By providing the distribution channel systems at different distancesfrom the centre axis of the die, it is also possible to provide withinthe die drainage conduits for draining extruded material leaking fromthe distribution channels.

U.S. patent specification No. 3,698,988 discloses an extrusion die forthe extrusion of a tubular film on a preformed tubular body. The priorart extrusion die which is no coextrusion die comprises a supply conduitfor supplying a plastics material to a first arched distribution channelwhich is provided in a plate member. Each end of said first distributionchannel is connected to a second arched distribution channel provided inan adjacent plate.

The ends of said latter distribution channel are connected to an annularinner distribution channel provided within the same plate and having adiameter which is less than that of the first and second distributionchannels. Thus, the flow of material from said arched distributionchannels to said annular distribution channel which opens into anannular extrusion die to a radial flow and not an axial flow.

The part of the extrusion die of the invention in which the distributionchannels are provided preferably consists of elements which are clampedtogether and the arched channels are preferably formed from groovesprovided in the surfaces of said elements.

An extrusion die composed of such elements presents several advantagesas far as its manufacture is concerned. Thus, it is much simpler to formgrooves in the surface of such elements than to form internal channelstherein. Furthermore, it is easy to maintain and clean such a diebecause the elements can easily be replaced, and when the die has beendisassembled the grooves forming the distribution channels are easilycleaned.

The arched distribution channels and the connecting channels connectingthe ends of one arched channel with the middle portions of the followingchannels may have any cross-sectional shape. In practice, however, ithas been found preferable to use channels having an essentiallyrectangular cross-sectional shape with rounded corners in order toobtain a uniform flow through said channels and to prevent a thermalconversion of slowly flowing material.

An embodiment of the extrusion die of the invention comprises archeddistribution channels each formed from two grooves, one groove beingformed in the surface of one element and the other in the surface of theadjacent element. This embodiment is particularly advantageous becauseit allows distribution channels having circular cross-section or havingrounded corners to be formed in a simple manner.

In another preferred embodiment of the die of the invention the elementsare disc-shaped and the arched channels are interconnected throughconnecting channels extending through the intermediate discs. Suchdisc-shaped elements are particularly preferred because they can beclamped together to form distribution channels without dead zones, evenif the elements are warped e.g. as a result of a high temperaturecleaning of the distribution channels.

The elements are preferably discs which extend perpendicular to the axisof the extrusion die and in that case the connecting channels preferablyare parallel to said axis.

The portion of the extrusion die comprising the distribution channelsystem may also be composed of shell-shaped elements inserted in oneanother and the channels connecting the arched channels are preferablyformed from grooves formed in the surfaces of the shell-shaped elements.Such a die is easily manufactured because all distribution channels ofthe distribution channel system can be formed by cutting.

A particularly uniform distribution of the extrudable material in theannular collecting chambers can be achieved by providing guide groovesat the points where the distribution channels open into the collectingchambers, said guide grooves being inclined relative to the centre axisof the die and the depth of said grooves gradually decreasing in thelongitudinal direction of the die.

Due to the presence of said guide grooves the extrudable materials aresubjected to a transverse movement after passing into the collectingchambers and said transverse movement contributes to the elimination orreduction of irregularities in the tubular bodies formed in thecollecting chambers.

A similar effect is achieved by using an extrusion die in which at leastone chamber wall in each collecting chamber is rotatable relative to theopenings through which the distribution channels open into thecollecting chamber considered.

The number and the length of the arched channels are preferably selectedsuch that the channels through which a given distribution channel systemis connected to a collecting chamber is uniformly distributed over theperiphery of the collecting chamber. In this manner the extrudablematerial supplied to said collecting chamber is uniformly distributedover the whole periphery of the tubular film formed in the extrusionslot.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic perspective view of an embodiment of anextrusion die according to the invention fed by three extruders,

FIG. 2 is a longitudinal sectional view of the extrusion die of FIG. 1taken along a radial plane passing through the line 11--11 of FIG. 1,

FIG. 3 is a part of the distribution channel system of the extrusion dieof FIG. 2 shown in unfolded view, and

FIG. 4 is a longitudinal sectional view of part of another embodiment ofthe extrusion die according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a coextrusion die 1 which through supply conduits isfed by three extruders 3. Each extruder 3 comprises a supply funnel 4for extrudable material. The extrusion die 1 has an annular extrusionslot 5 for the extrusion of a tubular film 6.

FIGS. 2 and 3 shows the extrusion die of FIG. 1 in enlarged scale. Thesupply conduit 2 which has one end inserted in a hole 10 in the end ofthe extrusion die 1 communicates with the middle of the threedistribution channel systems through a channel 11. The rear end of theextrusion die to which the extrudable materials is fed is composed ofaxially stacked plate elements 12-15 which are clamped together and alsoattached to the front end 16 of the extrusion die by clamping bolts 17which are screwed into threaded holes in front end 16. Three collectingchambers 18-20 are provided in said front end, one end of eachcollecting chamber communicating with a distribution channel system andthe opposite ends of collecting chambers 18-20 being connected to theannular extrusion slot 5.

The location of the extrusion slot 5 and consequently the thickness ofthe extruded tubular film is adjustable by means of adjusting screws 21and 22.

In the embodiment illustrated in FIG. 2 the extrusion die is providedwith electric heating jackets 23 on both the exterior and interiorsurfaces thereof.

The three distribution channel systems are in principle identical andonly the middle system will be described with reference to FIG. 3.

The channel 11 connecting the supply conduit 2 with the distributionchannel system opens into the middle portion of a first archeddistribution channel 24. Each end of channel 24 is connected to themiddle portion of a second group of arched distribution channels 25. Theends of channels 25 are in turn connected to the middle portion of athird group of arched distribution channels 26. The ends of thedistribution channels are connected to the collecting chamber 19 throughconnecting channels 27.

In order to obtain a good distribution of the partial streams leavingthe channels 27, guide grooves are provided in the wall of thecollecting chamber 19, these guide grooves 27 being inclined relative tothe longitudinal axis of the die and having a depth which graduallydecreases.

The extrusion die illustrated in FIG. 4 consists of a stationary partshown generally as 30, two rotatable parts 31 and 32 and located betweenparts 31 and 32 three collecting chambers 33, 34 and 35, each beingconnected to an annular extrusion slot 36.

The rotatable part 32 is mounted on a central shaft 37 cooperating withdriving means (not shown) mounted in a central bore in the stationarypart 30 of the die. The exterior surface of the rotatable part 31 hasthe shape of a toothed wheel which is in engagement with a pinion (notshown).

A guide ring 39 with a washer 40 attached to the stationary part 30 isinserted in an annular guide groove 41 in the rotatable part 31.Similarly a guide ring 42 with washer 43 which are also attached to thestationary part 30 are inserted in an annular guide groove 44 in therotatable part 32.

The distribution channel system of the die illustrated in FIG. 4corresponds to that of FIGS. 2 and 3 but the arched distributionchannels 24, 25 and 26 are composed of grooves which are provided in theconical surfaces of an inner element 45 and two surrounding conicalshells 46 and 47 as well as an outer shell 48 which has a conical innersurface, and a channel 49 connecting a supply conduit 2 with the firstarched channel 24.

The elements 45-48 containing the distribution channel system areclamped together by a plate 51 comprising a set of holes 50 and bolts 52which are inserted in said holes 50 and attached to the elements 45-48.Due to the conical shape of the contact surfaces of the elements 45-48only a small force and consequently relatively thin and short bolts arerequired to clamp together the portions forming the stationary part ofthe die.

The space between the outer rotatable part 31 and the inner rotatablepart 32 comprises two elements 53,54 of which one 53 by means offastening means (not shown) having a small cross-sectional area isconnected to the rotatable part 31 and the second one 54 by means ofsimilar fastening means (not shown) is connected to the other rotatablepart 32.

When using the embodiment of the die illustrated in FIGS. 2 and 3, thestreams flowing out of the channels 27 are smeared out during thepassage through the zone in which the guide grooves 28 are provided.

In the embodiment illustrated in FIG. 4 the streams are smeared out as aresult of the movement of the rotatable parts 31, 32. These parts can berotated in opposite directions (contrarotated) or in the same directionat different or equal speeds of rotation. Alternatively, one part can berotated while the other part is stationary.

It should be understood that the embodiment illustrated in FIGS. 2 and 3also might comprise rotatable die parts and that guide grooves alsomight be provided in walls of the collecting chambers illustrated inFIG. 4.

The embodiments of the extrusion die shown in the drawings are designedfor the manufacture of laminates consisting of three layers disposedparallel to the surfaces of the laminates. However, if the extrusion dieshown is modified such that one or two distribution channels systemsopen into the same collecting chamber, products could be manufacturedwherein segments of the extrudable materials are located side by side inone or more layers or extending from one surface to the opposite. Such amodified embodiment of the extrusion die according to the inventionwould be suitable for the manufacture of products of the type disclosedin U.S. Pat. Nos. 3,565,744, 3,505,162, 3,632,282 and 3,690,982 and partof the extrusion die of the invention which comprises the distributionchannel system could be combined with the apparatus described in saidU.S. patent specifications.

We claim:
 1. An extrusion die for the coextrusion of plural plasticmaterials into the form of a tubular film, said die comprising: (A)means for separately supplying at least two extrudable plastic materialsin a fluid state; (B) a material distribution section comprising anarray of annular elements providing a plurality of pairs of mutuallycontacting surfaces, at least one of said surfaces of each pair beingrecessed to define a labyrinthine distribution channel system for eachsuch material, each said system having an inlet opening connected to oneof said supply means and dividing into generally oppositely directedperipherally extending passages terminating in axially extendingconnecting passages which in turn divide into generally oppositelydirected peripherally extending passages terminating in axiallyextending connecting passages which in turn divide into oppositelyperipherally extending passages terminating in axially connectingpassages, the number of said passage divisions being sufficient toprovide for each extrudable material an array of at least eightperipherally spaced apart connecting passages opening generally axiallythrough an end wall of said distribution section, the pluraldistribution channel systems being arranged in radially displacedrelation at different radial distances from the center axis of theextrusion die; (C) an extrusion die section having one end wallcontacting said distribution section end wall and comprising a pluralityof concentric generally annular elements including generally spacedparallel concentric surfaces adjacent pairs of which define at least oneaxially elongated generally annular collecting chamber in communicationwith the open ends of the arrays of said connecting passages from saiddistribution sections, at least one of said annular elements definingsaid collecting chamber being mounted for rotation about its axisrelative to said material distribution section and relative to at leastthe other of said annular elements defining said chamber, a commoncontinuous annular extrusion orifice opening through the opposite endwall of said die section and connecting with the downsteam end of eachsuch annular collecting chamber; (D) mechanical engagement means unitingthe annular elements of said distribution and extrusion die sections,respectively, and maintaining said sections together and at least one ofsaid annular sections free for said relative rotation with said endfaces in contact; and (E) means for rotating said at least one rotatablymounted annular element of said die section about its axis relative tosaid distribution section and relative to said at least one otherannular element.
 2. An extrusion die as in claim 1 wherein said parallelconcentric surfaces of said concentric generally annular elements insaid die section define a plurality of said collecting chambers, one foreach of said extrudable materials, which chambers have their interiorends opening through said die section end wall in direct communicationwith the open ends of the corresponding array of connecting passagesfrom said distribution sections, and including delivery passage meansconnecting the downstream ends of said plural collecting chambers withsaid common extrusion orifice.
 3. The extrusion die of claim 2 whereinsaid delivery passage means merges the ends of said plural collectingchambers upstream of said extrusion orifice to form a common slot-likepassageway stretching from said merger point to said extrusion orifice.4. An extrusion die as in claim 1, characterized in that thedistribution channel systems are disposed in concentric relationship. 5.An extrusion die as in claim 1, characterized in that said distributionsection is composed of annular elements which are clamped together(12-15, 45-48) and that both surfaces of said elements are recessed todefine said passages.
 6. An extrusion die as in claim 5, characterizedin that the elements (12-15) are disc-shaped and that the axial passages(27) extend through the intervening dis-shaped elements (12-15) whilesaid peripheral passages are formed in at least one face thereof.
 7. Anextrusion die as in claim 5, characterized in that said part of the diecomprises annular frusto-conical elements (45-48) nestled together andthat the distribution passages are formed from recesses provided in atleast one surface of each frusto-conical element.
 8. An extrusion die asin claim 5, characterized in that at least one wall of each collectingchamber (33-35) is rotatable relative to the distribution section.
 9. Anextrusion die as in claim 1 including sealing means disposed between theend walls of said distribution and die sections to permit relativerotation to said rotatable element.
 10. An extrusion die as in claim 7wherein said mechanical engagement means comprises a common supportingplate for said annular frusto-conical elements disposed at one endthereof and means for anchoring said elements to said plate in nestledrelation.